1. Field of the Invention
The present invention is directed to integrated circuits containing multiple conductive metallization lines that are spaced apart from each other by polyimide insulating layers.
2. State of the Art
Integrated circuit fabrication requires the formation and patterning of conductive metallization lines, typically aluminum or aluminum silicon, to define inter-connections between various circuit elements formed in/on the device substrate. For products having a high density of such circuit elements, such as very large scale integration (VLSI) devices, it is normally necessary to form multiple metallization lines which are vertically spaced apart and separated by intermediate insulating layers. Specifically, it is well known that if individual metallization lines cross (contact) each other, then the crossing lines will short out. Accordingly, insulating layers are employed to vertically separate such individual lines thereby preventing their contact.
The insulating layers in the above-described integrated circuits are usually formed from silicon dioxide, although polyimide insulating layers offers a number of advantages over silicon dioxide layers. For example, one problem encountered in the formation of multiple metallization lines vertically spaced apart from each other is the propagation of surface irregularities. Specifically, the patterning of each metallization line results in an uneven surface whose contours are imparted to the overlying silicon dioxide insulating layer. Since integrated circuit fabrication requires substantial planarity of the insulating layer prior to addition of the next metallization line(s), additional steps are required to planarize the surface of the silicon dioxide insulating layer prior to formation of the next metallization line(s).
It is well known, however, that an advantage of polyimide insulating layers over silicon dioxide insulating layers is that the polyimide layers smooth over the irregularities of the underlying metallization lines, providing a superior surface for applying additional metallization lines. Additionally, the polyimide insulating layers are also free from discontinuities, such as small holes, which are frequently found in silicon dioxide insulating layers. Accordingly, the use of polyimides as insulating materials between metallization lines has been suggested.
Notwithstanding the above-noted benefits of polyimides insulating layers in integrated circuits, the commercial use of such layers has been significantly deterred by the fact that during fabrication and use, polyimides can release water. The released water can react with one or more of the components found in the integrated circuit (e.g., PO.sub.2 or chlorine) and generate species (e.g., acids such as H.sub.2 PO.sub.4 and HCl) that can corrode the metallization lines (i.e., corrodible species). Corrosion of the metallization lines by water released from the polyimide insulating layers leads to integrated circuits with substantially reduced useful lifetimes as compared to similar integrated circuits employing silicon dioxide in the insulating layers.